1. Field of the Invention
The present invention relates to a regenerative control circuit which controls regenerative energy of an electric motor drive system, and more particularly to a regenerative control circuit which operates to maintain a converter voltage constant even in the case of a large leakage current.
2. Description of the Prior Art
Generally, an electric motor drive system is provided with a control circuit which controls, upon stoppage of the motor, the regenerative energy produced by the armature of the motor during the period within which the motor is brought to a complete stop.
FIG. 1 shows a schematic diagram of the conventional regenerative control circuit for an electric motor drive system, and FIG. 2 shows a schematic diagram in relation to the routes of leakage currents in the regenerative control circuit. In the figure, numeral 1 designates a 3-phase a.c. power source, 2 a main rectifying circuit consisting of six diodes connected to the 3-phase a.c. power source 1, 3 a main smoothing capacitor connected to the output of the main rectifying circuit 2, 4 an inverter having its input connected to the main smoothing capacitor 3, 5 an electric motor connected to the output of the inverter 4, 6 a regenerative reference voltage rectifying circuit consisting of three diodes connected to the 3-phase a.c. power source 1, 7 a regenerative reference voltage smoothing capacitor connected between the positive electrode of the main smoothing capacitor 3 and the regenerative reference voltage rectifying circuit 6, 8 a regenerative energy dissipating resistor, and 9 a switching transistor for regeneration having its collector connected in series with the regenerative energy dissipating resistor 8. The regenerative energy dissipating resistor 8 and the regeneration transistor 9 are connected in parallel to the main smoothing capacitor 3. Numeral 10 designates a comparator which has one input connected to the node N' between the regenerative reference voltage rectifying circuit 6 and the regenerative reference voltage smoothing capacitor 7 and another input connected to the negative electrode of the main smoothing capacitor 3 through a regeneration d.c. voltage source 11. The output terminal of the comparator 10 is connected to the base of the regeneration transistor 9.
Next, the operation of the above circuit arrangement will be described. When the motor 5 is operated to decelerate, a flow of regenerative energy of the motor 5 appears in the route 20 as shown in FIG. 2, and the regenerative energy returning to the inverter 4 raises the terminal voltage of the main smoothing capacitor 3. On the other hand, the regenerative energy does not flow into the regenerative reference voltage smoothing capacitor 7, since its negative terminal is not connected to the inverter 4, and therefore the terminal voltage of the smoothing capacitor 7 does not vary. The main smoothing capacitor 3 and the regenerative reference voltage, smoothing capacitor 7 have their positive terminals connected together, causing the rising voltage of the main smoothing capacitor 3 to appear between the negative terminal node N' of the regenerative reference voltage smoothing capacitor 7 and the negative terminal node N of the main smoothing capacitor 3. The comparator 10 monitors this voltage, and it turns on the regeneration transistor 9 when the voltage rise has exceeded the voltage of the regeneration d.c. voltage source 11 so that the regenerative energy is dissipated by the regenerative energy dissipating resistor 8, thereby maintaining the voltage of the main smoothing capacitor 3 constant in carrying out the regenerative control.
In the foregoing conventional regenerative control circuit for an electric motor drive system, the regenerative energy dissipating resistor 8 and regeneration transistor 9 in serial connection are connected in parallel to the main smoothing capacitor 3, the negative terminal of the main smoothing capacitor 3 is connected to one input of the comparator 10 through the regeneration d.c. voltage source 11, the negative terminal of the regenerative reference voltage smoothing capacitor 7 is connected to another input of the comparator 10, and the output of the comparator 10 is connected to the base of the regeneration transistor 9, and in operation the voltage between the main smoothing capacitor 3 and the regenerative reference voltage smoothing capacitor 7 is maintained constant with respect to the power voltage. When the motor 5 is operated to decelerate, its regenerative energy causes only the terminal voltage of the main smoothing capacitor 3 to rise and, since these capacitors 3 and 7 have their positive electrodes connected together, the voltage rise of the main smoothing capacitor 3 is a voltage difference N'-N between the capacitors 3 and 7. This differential voltage is delivered to the comparator 10, which produces an output to turn on the regeneration transistor 9 so that the regenerative energy is dissipated by the regenerative energy dissipating resistor 8, thereby maintaining the voltage of the main smoothing capacitor 3 constant. The inverter 4 operates in a high-frequency switching mode, and a stray capacitance 12 existing in the motor 5 and on the motor power lines creates leakage currents on the routes indicated by 21 and 21' in FIG. 2. When the motor 5 is in the regenerative operation at deceleration, a leakage current also flows on the route through ground as shown by 22 in FIG. 2, and if this leakage current is large, the voltage of the regenerative reference voltage smoothing capacitor 7 rises, and a rise in the terminal voltage of the main smoothing capacitor 3 caused by the regenerative energy will not produce a voltage difference between the capacitors 3 and 7. Consequently, the comparator 10 will not produce the output signal, and the regeneration transistor 9 will not turn on, leaving the voltage of the main smoothing capacitor 3 to rise, resulting in the failure to maintain a constant capacitor voltage.